This final entry is something that "Lost" did so very well.
10. Make every character vulnerable, and not just the Nikkis and Paolos.
Nothing kills suspense in a scene like knowing that a character is bullet-proof. If a series is built around a few core characters, we're pretty sure that those characters are going to survive. Back when the Cartwrights road the range, we weren't all that concerned when Little Joe was shot.
"Lost" shook up that sense of security. Near the end of Season One, a major character is injured and dies. This adds fuel to the rivalry between Jack, the leader who is a man of science, and John, the leader who is a man of faith. It also means that everyone is in jeopardy. Now when one of the group is in harm's way, we get nervous for them.
Throughout the series, other major characters died. Each was wrenching and, I think, each was necessary to move the story forward. A few times newer characters that weren't working out were also killed, but those didn't have the impact of the others.
I'm often told I'm too nice to my characters. Why not? I like these people. But I'm learning to beat up on them for the sake of the story. Will I kill off any of my main characters? Don't know. It would certainly shake things up.
How about you? Have you every killed off a major character? What was the reason?